1. Field of the Invention
The invention relates generally to printing apparatus, and is particularly concerned with an offset printing apparatus employing an engraved printing plate and a deformable transfer pad made of silicone rubber or the like.
2. Description of the Prior Art
Pad transfer printing is a useful technique for printing on various types of surfaces, particularly raised surfaces and the surfaces of irregularly shaped objects. In this technique, the inked image is lifted from an engraved printing plate and is transferred to the surface to be printed by means of a resilient transfer pad, generally consisting of silicone rubber. The surface characteristics of the silicone rubber are such that the ink easily releases from the pad and preferentially adheres to the print receiving surface. The ability of the transfer pad to elastically deform during printing allows virtually any type of raised or irregularly shaped surface to be printed, in addition to flat surfaces. The process also lends itself to "wet on wet" printing of multicolor images, since the ink-repellent pad has no tendency to pick up a previously deposited ink image from the substrate when the next color is being printed.
Various types of automatic printing machines employing the pad transfer process have been developed. Genrally, these machines employ an engraved printing plate which is held in a face-up position at or near the base of the machine. The article to be printed is usually held at an adjacent work station with its print-receiving surface also facing upwardly. The transfer pad is held at an elevated position above the machine base and is arranged to move back and forth in a horizontal direction between the printing plate and the surface to be printed. At the end points of its horizontal travel, the transfer pad reciprocates vertically so as to be brought first into contact with the printing plate and then into contact with the print-receiving surface. The mechanical linkage which moves the transfer pad along the desired path is usually interconnected with a separate mechanism that is used for inking and scraping the printing plate, so that the plate is automatically re-inked and scraped during each printing cycle. The inking step involves flooding the printing plate with ink taken from a trough or reservoir by means of a brush, spreader blade, wire applicator, or the like. A doctor blade or other type of wiping or scraping device is then employed to remove excess ink from the plate, so that the ink remains only in the grooves or depressions which define the legend to be printed.
With presently available pad transfer printing machines, some difficulty is encountered in removing the printing plate for cleaning or legend changes. This difficulty arises in part from the fact that the printing plate is usually mounted on or near the base of the machine, beneath the pad mechanism and the linkages used for operating the inking and scraping devices. The operator must reach through or under these mechanisms in order to gain access to the printing plate, and, as a result, removal of the plate cannot be accomplished as quickly and conveniently as might be desired. Apart from the problem of access, however, there is a further difficulty in that the printing plate being removed will usually be covered with ink from the previous printing cycle. This is a consequence of the fact that most automatic pad transfer machines, when operating in an intermittent mode or when interrupted during continuous operation, are designed to stop at a predetermined home position in which the printing plate is left in a flooded condition (i.e., covered with ink but not yet scraped or doctored). If this were not so, residual ink left in the etched grooves of the plate would dry out if the machine were allowed to remain in the home position for an extended period of time without removing the plate. Hence it is necessary, when removing the printing plate for the purpose of cleaning or legend changes, to handle the plate in a flooded condition until the ink can be cleaned off. Even for experienced operators, it is difficult to remove the flooded plate without having the ink spill and soil the operator's hands, clothing and work area. This is disadvantageous not only because of the untidiness of the spilled ink, and the time and inconvenience involved in removing it, but also because a significant amount of ink is wasted. There is also a tendency for the ink on the surface of the plate to drip or spill onto the underlying portion of the machine base, on which the plate rests, as the plate is being removed. The ink which accumulates in this area can interfere with the proper seating of a new plate and can also impede later removal of the plate once the ink has dried. This situation can be remedied to some extent by cleaning the plate supporting area of the machine base after the plate is removed, but this creates additional inconvenience for the operator and increases the amount of time involved in changing the printing plate.